Muscles of the Trunk

Transversus perinei superficialis Ischiocavernosus
Bulbospongiosus Transversus perinei profundus
Sphincter urethrae

The superficial fascia of this region consists of a superficial, fatty, and a deeper membranous layer.

The fully layer is thick, loose, areolar in texture, and contains a variable amount of fat, in its meshes. In front, it is continuous with the dartos muscle of the scrotum; behind with the subcutaneous areolar tissue surrounding the anus; and, on each side, with the same fascia on the medial sides of the thighs. In the median plane, it is adherent to the skin and to the membranous layer of the superficial fascia.

The membranous layer of superficial fascia, (fig. 613) is thin, aponeurotic in structure, and of considerable strength, serving to bind down the muscles of the root of the penis. It is continuous, in front, with the dartos muscle, the deep fascia of the penis, and the membranous layer of the superficial fascia upon the anterior wall of the abdomen; on each side it is attached to the margins of the rami of the pubis and ischium, lateral to the crux penis and as far back as the tuberosity of the ischium; posteriorly it curves round the Transversi perinei superficiales to join the posterior margin of the perineal membrane (inferior fascia of the urogenital diaphragm) and the perineal body (central point of the perineum). In the median plane it is connected with the superficial fascia and with the median septum of the Bulbospongiosus. At its posterior part this fascia sends upwards from its deep surface a median septum, which incompletely divides the posterior portion of the subjacent space.

The perineal body (central tendinous point of the perineum).-This is a fibromuscular node in the median plane, about 1.25 cm. in front of the anus, and close to the bulb of the penis (urethral bulb). Towards this point six muscles converge and are attached: viz. the Sphincter ani externus, the Bulbospongiosus, the two Transversi perinei superficiales, and the anterior fibers of the two Levatores ani. In addition, it receives longitudinal involuntary fibers from the anterior aspect of the rectal ampulla and of the anal canal. It is a compact little node, and the importance of its integrity to the pelvic floor in the female has already been mentioned.

The Transversus perinei superficialis is a narrow muscular slip, which passes more or less transversely across the perineal space in front of the anus. It is often feebly developed, and is sometimes absent. It arises by tendinous fibers from the medial and anterior part of the tuberosity of the ischium, and, running medially, is inserted into the perineal body, joining in this situation with the muscle of the opposite side, with the Sphincter ani externus behind, and with the Bulbospongiosus in front. In some cases; the fibers of the deeper layer of the Sphincter ani externus decussate in front of the anus and are continued into this muscle. Occasionally it gives off fibers which join with the Bulbospongiosus of the same side.

Nerve-supply.-The Transverses perinei superficialis is supplied by the perineal branch of the pudendal nerve.

Action.-The simultaneous contraction of the two Transversi perinei superficiales helps to fix the perineal body.

Figure 615
Male pelvis muscles inferior view - Figure 615
The Bulbospongiosus (Bulbocavernosus) is placed in the median line of the perineum, in front of the anus, and consists of two symmetrical parts, united by a median tendinous raphe. It arises from this median raphe and from the perineal body (central tendinous point of the perineum). Its fibers diverge like the barbs of a quill-pen; the most posterior form a thin layer, which is lost on the perinea) membrane (inferior fascia of the urogenital diaphragm); the middle fibers encircle the bulb and the adjacent part of the corpus spongiosum penis (corpus cavernosum urethrae), and are inserted into a strong aponeurosis on the upper part of that structure; the anterior fibers spread out over the side of the corpus cavernosum penis, to be inserted partly into that body, anterior to the Ischiocavernosus, and partly into a tendinous expansion which covers the dorsal vessels of the penis.

Nerve-supply.-The Bubospongiosus is supplied by the perineal branch of the pudendal nerve.

Actions.–The Bulbospongiosus serves to empty the canal of the urethra after the bladder has expelled its contents; during the greater part of the act of micturition its fibers are relaxed, and they only come into action at the end of the process. The middle fibers are supposed by Krause to assist in the erection of the corpus spongiosum penis, by compressing the erectile tissue of the bulb. The anterior fibers, according to Tyrrel also contribute to the erection of the penis by compressing the deep dorsal vein of the penis as their tendinous expansion is inserted into, and is continuous with the fascia covering the dorsal vessels of the penis.

The Ischiocavernosus covers the crus penis. It arises by tendinous and fleshy fibers from the inner surface of the tuberosity of the ischium, behind the crus penis; and from the ramus of the ischium on both sides of the crus. The muscular fibers end in an aponeurosis which is inserted into the sides and under surface of the crus penis.

Nerve-supply.-The Ishiocavernosus is supplied by the perineal branch of the pudendal nerve.

Action.–The Ischiocavernosus compresses the crus penis, and retards the return of the blood through the veins, and thus serves to maintain the penis erect. Between the muscles just examined a triangular space exists, bounded medially by the Bulbospongiosus laterally by the Ischiocavernosus, and behind by the Transversus perinei superficialis; the floor is formed by the perineal membrane (inferior fascia of the urogenital diaphragm). Running from behind forwards in the space are the scrotal vessels and nerves, and the perineal branch of the posterior femoral cutaneous nerve; the transverse perineal artery courses along its posterior boundary on the Transversus perinei superficialis.

The deep fascia of the urogenital region forms an investment for the Transversus perinei profundus and the Sphincter urethra, but within it there are also the deep vessels and nerves of this part. It is stretched almost horizontally across the pubic arch, so as to close the anterior part of the pelvic outlet. It consists of two membranous laminae (fig. 616), which are united at the free edges of the muscles. The stronger and more superficial of these laminae named the perineal membrane (inferior fascia of the urogenital diaphragm). Its base, directed backwards, is connected to the perineal body and is continuous with the anal fascia, and, behind the Transversus perinei superficialis, with the membranous layer of the superficial fascia. Its lateral margins are attached to the inferior ramus of the pubis and the ramus of the ischium, above the crus penis. Its apex, directed forwards, is thickened to form the transverse perineal ligament; between this ligament and the inferior pubic ligament the deep dorsal vein of the penis (or clitoris) enters the pelvis. It is perforated, from 2 to 3 cm. below the symphysis pubis, by the urethra, the aperture for which is circular and about 6 mm in diameter; by the arteries and nerves to the bulb and the ducts of the bulbo-urethral glands close to the urethra; by the deep arteries of the penis, one on each side close to the pubic arch and about halfway along the attached margin of the membrane; by the dorsal arteries and nerves of the penis near the apex of the membrane. Its base is also perforated by the scrotal vessels and nerves.

If the perineal membrane be detached the following structures will be exposed : the membranous portion of the urethra, the Transversus perinei profundus and Sphincter urethra, the bulbo-urethral glands and their ducts, the pudendal vessels and dorsal nerves of the penis, the arteries and nerves of the bulb of the penis, and a plexus of veins.

These structures are separated .from the pelvis by a second, but less definite layer of fascia, which forms the deeper of the two lamina of the deep fascia of the urogenital region. It is continuous with the obturator fascia and stretches across the pubic arch. If the obturator fascia be traced medially after leaving the Obturator internus muscle, it will be found attached by some of its anterior fibers to the inner margin of the pubic arch, while its posterior fibers pass over this attachment to become continuous with the fascia on the deep surface of the Sphincter urethrae and the Transversus perinei profundus. Behind, this layer of the fascia blends with the perineal membrane, the perineal body and the membranous layer of the superficial fascia; above at the point where it is pierced by the urethra, it becomes continuous with the fascial sheath of the prostate.

The Transversus perinei profundus arises from the rami of the ischium and runs to the median line, where it interlaces in a tendinous raphe with its fellow of the opposite side. It lies in the same plane as the Sphincter urethrae; formerly the two muscles were described together as the Constrictor or Compressor urethrae.

Nerve-supply.-The Transversus perinei profundus is supplied by the perinea l branch of the pudendal nerve.

Action.-The Transversus perinei profundus is a tensor of the perineal body (central point of the perineum).

The Sphincter urethra surrounds the membranous portion of the urethra; and lies between the two layers of the deep fascia of the urogenital region. Its external fibers arise from the junction of the rami of the pubis and ischium, to the extent of 1.25 or 2 cm., and from the neighboring fascia;. They arch across the front of the urethra and bulbo-urethral glands, pass round the urethra, and behind it unite with the muscle of the opposite side, by means of a tendinous raphe. Its innermost fibers form a continuous circular investment for the membranous urethra.

Figure 616
Male pelvis, prostate coronal section - Figure 616
Nerve-supply.-The Sphincter urethrae is supplied by the perineal branch of the pudendal nerve.

Actions.-The muscles of both sides act together as a sphincter, compressing the membranous portion of the urethra. During micturition they, like the Bulbospongiosus, are relaxed, and only come into action at the end of the process to eject the last drops of urine.

 


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